This invention relates to a dual density non-woven batt. The batt is characterized by having a relatively high density layer and a relatively low density layer which renders it suitable for uses such as mattresses, furniture upholstery and similar applications where variable density and resistance against compression is desired, together with substantial resilience which will return the batt to its shape and thickness after compression for an indefinite number of cycles.
There are a number of advantages to be achieved by construction of batts for use as mattresses and upholstery from synthetic, staple fiber material. Such fibers are inherently lightweight and therefore easy to ship, store and manipulate during fabrication. These fibers are also generally less moisture absorbent than natural fibers such as cotton, or cellulosic based synthetic fibers such as rayon. Therefore, products made from these fibers can be maintained in a more hygienic condition and dried with much less expenditure of energy. Many such fibers also tend to melt and drip rather than burn. While some of these fibers give off toxic fumes, the escape of such fumes can be avoided or minimized by encapsulating the batt in a fire retardant or relatively air impermeable casing. In contrast, fibers such as cotton burn rapidly at high heat and generate dense smoke.
However, synthetic staple fibers also present certain processing difficulties which have heretofore made the construction of a relatively dense non-woven batt from synthetic staple fibers difficult and in some cases impractical. For example, the resiliency inherent in synthetic fibers such as nylon and polyester is caused by the plastic memory which is set into the fiber during manufacture. By plastic memory is meant simply the tendency of a fiber to return to a given shape upon release of an externally applied force. Unless the plastic memory is altered by either elevated temperature or stress beyond the tolerance of the fiber, the plastic memory lasts essentially throughout the life of the fiber. This makes formation of a batt by compressing a much thicker, less dense batt very difficult because of the tendency of the fibers to rebound to their original shape. Such fiber batts can be maintained in a compressed state, but this has sometimes involved the encapsulation of the batt in a cover or container. All of these methods create other problems such as unevenness and eventual deterioration of the batt due to fiber shifting, breakage and breakdown of the mechanical structure which maintains the compressed batt.
Batts formed of constituent parts having varying densities are known in the prior art. However, none are known in which both parts remain highly compressible. For example, one known product comprises a relatively thick batt layer onto which is secured in some manner a very thin, compressed, almost cardboard-like layer of fibers which cannot be further compressed to any significant degree. This thin layer is a very small fraction of the thickness of the batt.
Several prior art processes can be used to compress the batt into its desired density, such as by use of engaging members such as rollers or plates on both sides of the batt. In effect, the batt is heated simultaneously from both sides to the point where its elastic memory is relaxed. However, the batt must then be removed from the rollers, plates or the like which have held the batt in its compressed state. Even with the use of TFE or other similarly coated rollers or plates, sticking is a common problem. In addition, even heating is inherently difficult to obtain since the fibers in contact with the heated metal surfaces are heated almost instantly whereas fibers in the interior of the batt are heated at a much slower rate. If the rollers between which the batt is traveling ar heated to the extent necessary to completely relax the plastic memory of the fibers on the interior of the batt, quite often the fibers in intimate contact with the rollers will melt completely or disintegrate. If the rollers are cooled to avoid complete melting of the fibers on the outer surface of the batt, the interior fibers are not heated sufficiently to reset their plastic memory. In this event, the outer fibers are constantly being pushed against from the interior by fibers whose plastic memory is constantly attempting to cause the fibers to reassume their original shape. Attempts to correct this problem have included varying the percentage of fibers having relatively different melting temperatures through the cross-section of the batt or providing fibers on the interior of the batt having a relatively lower temperature at which the elastic memory is relaxed.
Prior patent numbers 4668562 and 4753693, both owned by applicant here, disclose the use of a sufficient vacuum suction through a perforated roller to draw the batt down into its desired thickness during the heating and compression stages of processing. The compressed batt is cooled and maintains the compressed thickness after cooling. The process disclosed in these patents is useful in the practice of the invention disclosed and claimed in this application and is disclosed here as the preferred embodiment for forming the dual density batt, although certainly not the only suitable means of doing so.